There have been recent findings on the mechanisms by which geckos adhere to and climb smooth vertical surfaces. Geckos are exceptional in their ability to climb up smooth vertical surfaces because their hierarchical micro/nanoscale foot-hairs with their spatulate tips can attach to almost any smooth or micro/nanoscale rough surface repeatedly with a controllable adhesion pressure up to around 10 N/cm2 (100 kPa) [K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full, Nature, 405, 681 (2000)]. Recent findings have shown that van der Waals and possibly capillary forces play a dominant role in their fibrillar adhesion [K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, and R. J. Full, Nature, 405, 681 (2000)] [K. Autumn, M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full, PNAS, 99, 12252 (2002)] [G. Huber, H. Mantz, R. Spolenak, K. Mecke, K. Jacobs, S, N. Grob, and E. Artz, PNAS, 102(45), 16293 (2005)] [G. Huber, S. N. Grob, R. Spolenak, and E Artz, Biology Letters, 1, 2 (2005)].
Many adhesion and contact mechanics models for the microfibrillar interfaces have been developed [H. Gao and H. Yao, PNAS, 101, 7851 (2004)] [T. Tang, C. Hui, and N. J. Glassmaker, J. Roy. Soc. Interface, 2, 505 (2005)] [N. J. Glassmaker, A. Jagota, C. Hui, and J. Kim, J. Roy. Soc. Interface, 1, 1 (2004)] [C. Hui, N. J. Glassmaker, T. Tang, and A. Jagota, J. Roy. Soc. Interface, 1, 35 (2004)] [B. N. J. Persson, J. Chemical Physics, 118, 7614 (2003)] [A. J. Crosby, M. Hageman, and A. Duncan, Langmuir, 21, 11738 (2005)] and synthetic fibrillar adhesives have been attempted to be fabricated. Fabrication methods for recent micro/nanoscale synthetic dry adhesives consist of electron-beam lithography [A. K. Geim, S. V. Dubnos, I. V. Grigorieva, K. S. Novoselov, A. A. Zhukov, and S. Y. Shapoval, Nature Materials, 2, 461 (2003)], replication of templates using molding or casting [D. Campolo, S. Jones, and R. S. Fearing, Proc. of the IEEE Nanotechnology Conf., 12 (2003)], drawing [H. E. Jeong, S. H. Lee, P. Kim, and K. Y. Suh, Nano Letters, 6, 1508 (2006)], printing [M. Sitti and R. S. Fearing, J. Adhesion Science and Technology, 17(8), 1055 (2003)], growing [Y. Zhao, T. Tong, L. Delzeit, A. Kashani, M. Meyyappan, and A. Majumdar, J. Vac. Sci. Techno. B, 24(1), 331 (2006)], and more complex microfabrication combined with self-assembly [M. T. Northen and K. L. Turner, Nanotechnology, 16, 1159 (2005)]. These works focused on fabricating micro/nanoscale high aspect ratio and high density polymer or carbon nanotube fibers on a flat substrate.
Some researchers have attempted to recreate the spatulate tips that occur naturally on gecko hairs as such broadened tips on fibers provide additional surface area, promoting adherence, while acting to prevent clumping of the fibers.
U.S. Pat. No. 6,722,026 discloses a method of removably adhering a semiconductor substrate with microfibers which possess spatulate tips, but does not disclose a method for fabrication of such spatulate tips. U.S. patent application Ser. No. 11/281,768 (published as US 2006-0202355 A1) discloses a variety of formulae for enhancing friction of fibers and mentions fibers with T-shaped ends, but does not describe a method of fabrication for such T-shaped ends.
U.S. Pat. No. 6,737,160 and U.S. patent application Ser. No. 10/039,574 (published as US 2003 0124312 A1), Ser. No. 10/655,271 (published as US 2005 0072509 A1), Ser. No. 10/747,923 (published as US 2005 0148984 A1) and Ser. No. 11/030,752 (published as US 2005 0151385 A1) describe the use of microfibers as dry adhesives, and describe several methods for the fabrication of spatulate tips on such microfibers. One method uses an oxide/nitride semiconductor process to fabricate the shafts, the ends of which are then roughened to produce spatula. This method can not make spatulate tips. Another described method uses a pipette, through which a liquid polymer is extruded until a hemispherical drop forms at the end of the pipette, which could then be flattened against a smooth surface to create a flat spatulate tip. This method can allow only micrometer scale fibers due to the diameter limitation of micro-pipettes. Another described method to fabricate spatulate tips is lithographically induced self-construction. This technique uses electrostatic attraction to pull liquid through a mask, to thereby ‘sprout’ spatulae. This could enable micro/nano-meter scale low aspect ratio fibers with no tips. A third method described to fabricate spatulate tips involves the use of a nano-imprinting roller. This could also enable micro/nano-meter scale low aspect ratio fibers with no tips. A final method uses a two-layer photoresist process to make fibers with tips. This method can only make fibers with tips from photoresist polymers, which are very brittle.
U.S. patent application Ser. No. 10/863,129 (published as US 2005-0271869-A1) and Ser. No. 10/982,324 (published as US-2005-0271870-A1) disclose a method for forming hierarchical structures of microfibers with smaller microfibrils attached to the end. In one embodiment, these applications describe a method to fabricate microstructures with broader tips on narrow shafts that could be considered to be spatulate tips. This method uses a time multiplexed deep etching process, such as the Bosch process to etch wells in a substrate. Through alternation of etching and passivation, the process can produce an array of microfibers with large heads on top of narrow shafts. This process can make microfibers with flat tips from only silicon type of stiff and brittle materials that can be etched in the Bosch process.
The microfiber fabrication methods described above are very expensive for producing commercial quantities of adhesive materials. Therefore there is a need for better methods for economically producing microfiber-based dry adhesives. Moreover, fibers with flat tips and diameters of hundreds of nanometer can not be reliably fabricated from wide range of polymer materials in the above processes.
Accordingly, there is a need for improved dry adhesives and improved methods for making dry adhesives. In particular, there is a need for dry adhesives having greater adhesive forces and improved durability. In addition, there is a need for methods of making dry adhesives with lower costs of production. Those and other advantages of the present invention will be described in more detail hereinbelow.